1. Field of the Invention
The present invention relates to the field of standard interfaces, and more particularly to interfaces which can transmit and receive data from other devices which may include devices operating from a different, particularly a higher voltage, power supply.
2. Prior Art
Interface standards such as RS485 and RS232 have been used for a long time and will continue to be used despite the drop in the standard power supply voltages below 5 V. The output swing of the RS485/232 standards do not allow the traditional output stages to be used at a supply voltage of 3.3 V .+-.0.3 V. Portable computers will be using only 3.3 V power supplies very soon, and they still need these interface chips to communicate with other computers and peripherals. A new output stage disclosed herein has been generated which allows the lower power supply voltage, yet meets these interface standards.
Two different traditional techniques are shown in FIG. 1 and 2. FIG. 1 illustrates a bipolar technique and FIG. 2 illustrates a CMOS technique. The bipolar technique has an output voltage high level of Vcc-[V.sub.ce1(sat) +V.sub.be2 ]. This typically comes out to be Vcc-1.05 V. The bipolar technique has an output voltage low level of V.sub.d5 +V.sub.ce3(sat). This typically comes out to be 0.8 V. The CMOS technique has an output voltage high level of Vcc-[V.sub.ds7 +V.sub.d6 ]. This typically comes out to be Vcc-0.8 V. The CMOS technique has an output voltage low level of V.sub.d9 +V.sub.ds8. This typically comes out to be 0.8 V. If these techniques were to be used at a supply voltage of 3.0 V, the bipolar differential output swing would be 3.0-(1.05+0.8)=1.15 V and the CMOS technique would be 3.0-(0.8+0.8)=1.4 V. RS485 standards require 1.5 V differentially across a 54 ohm load. RS422 requires 2.0 V (single-ended) across a 100 ohm load. Clearly neither technique satisfies either specification at the reduced supply voltage.
Each previous technique used a Schottky diode in series with an active device (NPN, PNP, PMOS or NMOS) to isolate the output from the power supplies. This allows the output to be pulled above or below the power supply by devices on the same bus operating from a higher voltage supply without driving current into either lower voltage supply. The biggest problem with using Schottky diodes is that their forward biased voltage is around 450 mV. In the bipolar case, the largest differential voltage (while using a 3.0 V supply) is [3.0-(0.35+0.7+0.45+0.35)]=1.15 V. This will not allow either RS485 or RS422 to be driven. In the CMOS case, the largest differential voltage (again while using a 3.0 V supply) is [3.0-(0.35+0.45+0.45+0.35)]=1.40 V. This will still not allow either RS485 or RS422 to be driven. Note this is true even if the active devices were made large (V.sub.ds or V.sub.ce &lt;0.35 V).